Does your project have military characters in it? If so, then I strongly recommend you know how to spell their ranks. I lost count long ago of the number of scripts that misspelled sergeant, colonel, and more. Some of those misspellings are truly inventive, but unfortunately, this is not the kind of creativity that you want to showcase.
You should also take a minute or two to familiarize yourself with how ranks work. Does it make sense to have a 58-year-old captain, or a 22-year-old Lt. Col.? Your character’s age and rank can be used to say a lot, whether it’s a rapid rise due to excellence or nepotism, or being passed over for promotion and consigned to a hellhole as punishment. Exploit this opportunity.
Do you know the difference between officers and NCOs? I read a military sci-fi piece where a senior officer got demoted to private. Now, in sci-fi, I suppose anything can happen, but in every universe your readers are familiar with, officers are officers and NCOs are NCOs. And for officers, as the old saying goes: there is nothing lower than a second lieutenant. While NCOs can be busted down in rank, officers get passed over for promotion or pushed out of the service.
Why does this matter? It’s about removing obstacles. If you’re writing a contemporary or historical military story and you get your ranks wrong, you run the risk of damaging your credibility. Even if your characters are cruising the stars to fight the intergalactic nemesis, why reinvent the ranking system? It takes time to explain, and is counterintuitive. You’re just creating speed bumps for your audience.
As always, do everything you can to smooth the road for your readers. Errors that take them out of the story you’re telling–whether it takes place at West Point or in a galaxy far, far away–needlessly undermine your work.